Dr. Brennan, who practices osteopathic medicine in Houston and is the founder of the International Academy of Preventive Medicine, is a hard-nosed opponent of sugar, processed flour and chemical additives. But he is not an organic food or vitamin crank and the mounting evidence tends to support his contention that an imbalance in the body's sugar-regulatory mechanism leads to both hypoglycemia (low blood sugar) and its opposite, diabetes. Indeed Brennan argues that every diabetic once suffered from hypoglycemia, a condition commonly misdiagnosed and mistreated because its symptoms--blurred vision, headaches, irritability, depression, fatigue--are both varied and diffuse. And since a six-hour glucose tolerance test is required to confirm this frequently sub-clinical malady, it is mostly ignored or treated with tranquilizers or other palliatives. Brennan suggests--quite convincingly--that although most Americans are well-informed on viral and bacterial diseases, they are far less knowledgeable about the long-range causes of the degenerative diseases and that sugar (""sugar, like heroin, should be banned"") contributes to the chemical disturbance of liver, pancreas, heart and other organs. Brennan contends that recent research has shown that all alcoholics and all asthmatics are hypoglycemic and that the treatment of mental illness with diet and nutritional supplements is only beginning. He's not in the mainstream of contemporary medicine but the emphasis on preventive approaches to disease does seem to be gathering more and more supporters.